Method and system for compromise greenfield preambles for 802.11n

ABSTRACT

Aspects of the invention described herein may enable a greenfield access mode in IEEE 802.11n WLAN systems in comparison to an alternative approach that may not provide greenfield access. The utilization of greenfield access may reduce the portion of time required to transmit data due to overhead comprising preamble fields and header fields. This may enable higher data throughput rates to be achieved. This may further enable more robust transmission of data by enabling comparable data rates to be maintained while reducing the coding rate of encoded transmitted data. The reduction of the coding rate may enable comparable data rates to be maintained for transmission via RF channels characterized by lower SNR while still achieving desired target levels of packet error rates. In another aspect of the invention, mixed mode access may be achieved while reducing the portion of time required for transmitting data due to overhead.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application makes reference, claims priority to, and claims thebenefit of U.S. Provisional Application Ser. No. 60/653,429 filed Feb.16, 2005.

This application makes reference to:

U.S. patent application Ser. No. 10/973,595 filed Oct. 26, 2004;

U.S. patent application Ser. No. 11/052,353 filed Feb. 7, 2005; and

U.S. patent application Ser. No. 11/052,389 filed Feb. 7, 2005.

All of the above state applications are hereby incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to wireless communication.More specifically, certain embodiments of the invention relate to amethod and system for compromise greenfield preambles for 802.11n.

BACKGROUND OF THE INVENTION

Within the IEEE organization, IEEE 802.11 task group N (TGn) has beenchartered to develop a standard to enable WLAN devices to achievethroughput rates beyond 100 Mbits/s. This standard may be documented inIEEE resolution 802.11n.

The Institute for Electrical and Electronics Engineers (IEEE), inresolution IEEE 802.11, also referred as “802.11”, has defined aplurality of specifications which are related to wireless networking.With current existing 802.11 standards, such as 802.11(a),(b),(g), whichmay support up to 54 Mbps data rates, either in 2.4 GHz or in 5 GHzfrequency bands. Within the IEEE organization, IEEE 802.11 task group N(TGn) has been chartered to develop a standard to enable WLAN devices toachieve throughput rates beyond 100 Mbits/s. This standard may bedocumented in IEEE resolution 802.11n. A plurality of proposals isemerging as candidates for incorporation in IEEE resolution 802.11n.Among them are proposals from TGn Sync, which is a multi-industry groupworking to define proposals for next generation wireless networks thatare to be submitted for inclusion in IEEE 802.11n. The proposals may bebased upon what may be referred as a “sounding frame”. The soundingframe method may comprise the transmitting of a plurality of longtraining sequences (LTSs) that match the number of transmitting antennaat the receiving mobile terminal. The sounding frame method may notutilize beamforming or cyclic delay diversity (CDD). In the soundingframe method, each antenna in a multiple input multiple output (MIMO)system may transmit independent information.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present invention asset forth in the remainder of the present application with reference tothe drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method for compromise greenfield preambles for 802.11n,substantially as shown in and/or described in connection with at leastone of the figures, as set forth more completely in the claims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary system for wireless datacommunications, which may be utilized in accordance with an embodimentof the invention.

FIG. 2 a is an exemplary block diagram of a transceiver which may beutilized in accordance with an embodiment of the invention.

FIG. 2 b is an exemplary block diagram of a transceiver comprising atransmitter and a receiver in a MIMO system, which may be utilized inaccordance with an embodiment of the invention.

FIG. 3 a illustrates an exemplary physical layer protocol data unit,which may be utilized in connection with an embodiment of the invention.

FIG. 3 b illustrates an exemplary data field in a PPDU, which may beutilized in connection with an embodiment of the invention.

FIG. 4 a shows exemplary training fields and header fields for mixedmode access in accordance with a TGn Sync proposal that may be utilizedin connection with an embodiment of the invention.

FIG. 4 b shows an exemplary L-SIG header field for mixed mode access inaccordance with a TGn Sync proposal that may be utilized in connectionwith an embodiment of the invention.

FIG. 4 c shows an exemplary HT-SIG header field for mixed mode access inaccordance with a TGn Sync proposal that may be utilized in connectionwith an embodiment of the invention.

FIG. 5 a shows exemplary training fields and header fields forgreenfield access in accordance with a WWiSE proposal for N_(ss)=2, inaccordance with an embodiment of the invention.

FIG. 5 b shows an exemplary Signal-N header field for greenfield accessin accordance with a WWiSE proposal, in accordance with an embodiment ofthe invention.

FIG. 5 c shows exemplary training fields and header fields forgreenfield access in accordance with a WWiSE proposal for N_(ss)=4, inaccordance with an embodiment of the invention.

FIG. 6 a shows exemplary training fields and header fields with trailingsignal field for greenfield access for N_(ss)>2, in accordance with anembodiment of the invention.

FIG. 6 b shows exemplary training fields and header fields with earlysignal field for greenfield access for N_(ss)>2, in accordance with anembodiment of the invention.

FIG. 7 shows exemplary training fields and header fields for mixed modeaccess for N_(ss)>2, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the invention relate to a method and system forcompromise greenfield preambles for 802.11n, which utilizes a channelsounding mechanism to communicate information between a transmitter anda receiver. Various embodiments of the invention may enable a greenfieldaccess mode in IEEE 802.11n WLAN systems compared to an alternativeapproach that may not provide methods for greenfield access. Theutilization of greenfield access may reduce the portion of time requiredto transmit data due to overhead comprising preamble fields and headerfields. This may enable higher data throughput rates to be achieved.This may further enable more robust transmission of data by enablingcomparable data rates to be maintained while reducing the coding rate ofencoded transmitted data. The reduction of the coding rate may enablecomparable data rates to be maintained for transmission via RF channelscharacterized by lower SNR while still achieving desired target levelsof packet error rates.

In another embodiment of the invention, mixed mode access may beachieved while reducing a portion of time required for transmitting datadue to overhead comprising preamble fields and header fields. Longtraining fields among a plurality of transmitted spatial streams maycomprise orthonormal long training sequences, which may obviate toneinterleaving. Utilizing orthonormal long training sequences may enablethe transmission of identical symbols via a plurality of spatialstreams.

FIG. 1 is a block diagram of an exemplary system for wireless datacommunications, which may be utilized in accordance with an embodimentof the invention. With reference to FIG. 1 there is shown a distributionsystem (DS) 110, an extended service set (ESS) 120, and an IEEE 802.xLAN 122. The ESS 120 may comprise a first basic service set (BSS) 102,and a second BSS 112. The first BSS 102 may comprise a first 802.11 WLANstation 104, a second 802.11 WLAN station 106, and an access point (AP)108. The second BSS 112 may comprise a first 802.11 WLAN station 114, asecond 802.11 WLAN station 116, and an access point (AP) 118. The IEEE802.x LAN 122 may comprise an 802.x LAN station 124, and a portal 126.

The BSS 102 or 112 may be part of an IEEE 802.11 WLAN that comprises atleast 2 IEEE 802.11 WLAN stations, for example, the first 802.11 WLANstation 104, the second 802.11 WLAN station 106, and the AP 108, whichmay be members of the BSS 102. Non-AP stations within BSS 102, the first802.11 WLAN station 104, and the second 802.11 WLAN station 106, mayindividually form an association with the AP 108. An AP, such as AP 108,may be implemented as an Ethernet switch, bridge, or other device in aWLAN, for example. Similarly, non-AP stations within BSS 112, the first802.11 WLAN station 114, and the second 802.11 WLAN station 116, mayindividually form an association with the AP 118. Once an associationhas been formed between a first 802.11 WLAN station 104 and an AP 108,the AP 108 may communicate reachability information about the first802.11 WLAN station 104 to other APs associated with the ESS 120, suchas AP 118, and portals such as the portal 126. In turn, the AP 118 maycommunicate reachability information about the first 802.11 WLAN station104 to stations in BSS 112. The portal 126, which may be implemented as,for example, an Ethernet switch or other device in a LAN, maycommunicate reachability information about the first 802.11 WLAN station104 to stations in LAN 122 such as the 802.x LAN station 124. Thecommunication of reachability information about the first 802.11 WLANstation 104 may enable WLAN stations that are not in BSS 102, but areassociated with ESS 120, to communicate with the first 802.11 WLANstation 104.

The DS 110 may provide an infrastructure which enables a first 802.11WLAN station 104 in one BSS 102, to communicate with a first 802.11 WLANstation 114 in another BSS 112. The DS 110 may also enable a first802.11 WLAN station 104 in one BSS 102 to communicate with an 802.x LANstation 124 in an IEEE 802.x LAN 122, implemented as, for example awired LAN. The AP 108, AP 118, or portal 126 may provide a means bywhich a station in a BSS 102, BSS 112, or LAN 122 may communicateinformation via the DS 110. The first 802.11 WLAN station 104 in BSS 102may communicate information to a first 802.11 WLAN station 114 in BSS112 by transmitting the information to AP 108, which may transmit theinformation via the DS 110 to AP 118, which in turn may transmit theinformation to station 114 in BSS 112. The first 802.11 WLAN station 104may communicate information to the 802.x LAN station 124 in LAN 122 bytransmitting the information to AP 108, which may transmit theinformation via the DS 110 to the portal 126, which in turn may transmitthe information to the 802.x LAN station 124 in LAN 122. The DS 110 mayutilize wireless communications via an RF channel, wired communications,such as IEEE 802.x Ethernet, or a combination thereof.

The IEEE resolution 802.11n may enable WLAN devices compatible with IEEE802.11in to also interoperate with IEEE 802.11 devices that are notcompatible with IEEE 802.11n. WLAN devices that are compatible with IEEE802.11 but are not compatible with IEEE 802.11 n may be referred to aslegacy IEEE 802.11 WLAN devices. WLAN devices that are compatible withIEEE 802.11n and communicate with other IEEE 802.11n compatible WLANdevices in an IEEE basic service set (BSS) of which no legacy IEEE802.11 WLAN devices are currently members may be capable ofcommunicating in a greenfield access mode. When utilizing greenfieldaccess, communications between the WLAN devices may utilize capabilitiesspecified in IEEE 802.11n that may not be accessible to legacy WLANdevices. WLAN devices that are compatible with IEEE 802.11n, and thatcommunicate with IEEE 802.11n compatible WLAN devices in an IEEE BSS, ofwhich legacy IEEE 802.11 WLAN devices are currently members, may utilizemixed mode access. When utilizing mixed mode access, IEEE 802.11ncompatible WLAN devices may utilize spoofing to avoid interference fromlegacy IEEE 802.11 WLAN devices during communications between IEEE802.11n compatible devices in a BSS.

Among proposals received by TGn are proposals from, the worldwidespectrum efficiency (WWiSE) group, and TGn Sync. Current proposals fromTGn Sync may not provide a mechanism to support greenfield access. Assuch, mixed mode access communications based on current TGn Sync may berequired to comprise information that may not be required in greenfieldaccess communications.

The WWiSE proposals may comprise a plurality of enhancements to legacyIEEE 802.11 WLAN devices for incorporation in IEEE 802.11n WLAN devices.Legacy IEEE 802.11 WLAN devices may utilize 20 RF MHz channels. IEEE802.11n may utilize 20 MHz channels, with an optional utilization of 40RF MHz channels. Legacy IEEE 802.11 WLAN devices may utilize 52 sub-bandfrequencies, or subcarriers, in a 20 MHz channel, comprising pilot tonesat 4 sub-band frequencies, and 48 data-bearing subcarriers. IEEE 802.11nWLAN devices based on WWiSE proposals may utilize a total of 56subcarriers in a 20 MHz channel, comprising 2 pilot tones, and 54data-bearing subcarriers. The subcarriers may be distributedsymmetrically around a frequency that comprises the center frequency ofa 20 MHz channel. The frequency spacing between subcarriers in an IEEE802.11n WLAN device may be approximately equal to 312.5 KHz. Therefore,an IEEE 802.11n 20 MHz channel may comprise a plurality of subcarriersfor which the frequency of a subcarrier, f_(sc)(i), may be representedas:f _(sc)(i)=f _(center) +iΔ _(f) where,   equation[1]the frequency, f_(center), may represent the center frequency in a 20MHz channel, the frequency increment, Δ_(f), may represent the frequencyspacing between subcarriers, and the value of the subcarrier index, i,may comprise a plurality of integer values represented as:0<i≦N _(sc)/2, or   equation[2a]−N _(sc)/2≦i<0, where   equation[2b]N_(sc) may represent the number of subcarriers present in a 20 MHzchannel.

An IEEE 802.11n 40 MHz channel may comprise a plurality of subcarriersfor which the frequency of a subcarrier f⁴⁰ _(sc)(i) may be representedas:f _(sc) ⁴⁰(i)=f _(primary) +iΔ _(f), or   equation[3a]f _(sc) ⁴⁰(i)=f _(secondary) +iΔ _(f), where   equation[3b]f_(primary) may represent the center frequency of a primary 20 MHzchannel, f_(secondary) may represent the center frequency of a secondary20 MHz channel, and the index, i, may be as defined in equations [3a and[3b]. The primary and secondary 20 MHz channels may be adjacent channelssuch that:f _(secondary) =f _(primary)±20 MHz, where   equation[4]the secondary 20 MHz channel may be located at an adjacent channel forwhich the center frequency f_(secondary) is either 20 MHz higher or 20MHz lower than the center frequency of the primary 20 MHz channelf_(primary). A 40 MHz channel may comprise a plurality of N_(sc)subcarriers located at the primary 20 MHz channel, and subsequentplurality of N_(sc) subcarriers located at the secondary 20 MHz channel,where N_(sc) may represent the number of subcarriers in a 20 MHzchannel. In this regard, a 40 MHz channel may comprise a total of2N_(sc) subcarriers. The state of the secondary 20 MHz channel may notbe evaluated during communications between IEEE 802.11n WLAN devices.

The WWiSE proposals may incorporate a plurality of MIMO antennaconfigurations represented as N_(TX)×N_(RX), where N_(TX) may representthe number of transmitting antenna at a station. Transmitting antennasmay be utilized to transmit signals via an RF channel. N_(RX) mayrepresent the number of receiving antenna at a station that receives thesignals transmitted by the N_(TX) transmitting antenna. The MIMO antennaconfiguration may enable IEEE 802.11n WLAN devices to achieve higherdata rates than legacy IEEE 802.11 WLAN devices. A legacy 802.11 WLANdevice may achieve data rates of 54 Mbits/s based on IEEE 802.11aspecifications. By comparison, an IEEE 802.11n WLAN device may achievedata rates of 540 Mbits/s in a 4×4 MIMO configuration.

FIG. 2 a is an exemplary block diagram of a transceiver which may beutilized in accordance with an embodiment of the invention. Withreference to FIG. 2 a is shown a baseband processor 272, a transceiver274, an RF front end 280, a plurality of receive antennas 276 a, . . . ,276 n, and a plurality of transmitting antennas 278 a, . . . , 278 n.The transceiver 274 may comprise a processor 282, a receiver 284, and atransmitter 286.

The processor 282 may be adapted to perform digital receiver and/ortransmitter functions in accordance with applicable communicationsstandards. These functions may comprise, but are not limited to, tasksperformed at lower layers in a relevant protocol reference model. Thesetasks may further comprise the physical layer convergence procedure(PLCP), physical medium dependent (PMD) functions, and associated layermanagement functions. The baseband processor 272 may be adapted toperform functions in accordance with applicable communicationsstandards. These functions may comprise, but are not limited to, tasksrelated to analysis of data received via the receiver 284, and tasksrelated to generating data to be transmitted via the transmitter 286.These tasks may further comprise medium access control (MAC) layerfunctions as specified by pertinent standards.

The receiver 284 may be adapted to perform digital receiver functionsthat may comprise, but are not limited to, fast Fourier transformprocessing, beamforming processing, equalization, demapping,demodulation control, deinterleaving, depuncture, and decoding. Thetransmitter 286 may perform digital transmitter functions that comprise,but are not limited to, coding, puncture, interleaving, mapping,modulation control, inverse fast Fourier transform processing,beamforming processing. The RF front end 280 may receive analog RFsignals via antennas 276 a, . . . , 276 n, converting the RF signal tobaseband and generating a digital equivalent of the received analogbaseband signal. The digital representation may be a complex quantitycomprising I and Q components. The RF front end 280 may also transmitanalog RF signals via an antenna 278 a, . . . , 278 n, converting adigital baseband signal to an analog RF signal.

In operation, the processor 282 may receive data from the receiver 284.The processor 282 may communicate received data to the basebandprocessor 272 for analysis and further processing. The basebandprocessor 272 may generate data to be transmitted via an RF channel bythe transmitter 286. The baseband processor 272 may communicate the datato the processor 282. The processor 282 may generate a plurality of bitsthat are communicated to the receiver 284.

FIG. 2 b is an exemplary block diagram of a transmitter and a receiverin a MIMO system, which may be utilized in accordance with an embodimentof the invention. With reference to FIG. 2 b is shown a transmitter 200a receiver 201, a processor 240, a baseband processor 242, a pluralityof transmitter antennas 215 a, . . . , 215 n, and a plurality ofreceiver antennas 217 a, . . . , 217 n. The transmitter 200 may comprisea coding block 202, a puncture block 204, an interleaver block 206, aplurality of mapper blocks 208 a, . . . , 208 n, a plurality of inversefast Fourier transform (IFFT) blocks 210 a, . . . , 210 n, a beamformingV matrix block 212, and a plurality of digital to analog (D/A)conversion and antenna front end blocks 214 a, . . . , 214 n. Thereceiver 201 may comprise a plurality of antenna front end and analog todigital (A/D) conversion blocks 216 a, . . . , 216 n, a beamforming Umatrix block 218, a plurality of fast Fourier transform (FFT) blocks 220a, . . . , 220 n, a channel estimates block 222, an equalizer block 224,a plurality of demapper blocks 226 a, . . . , 226 n, a deinterleaverblock 228, a depuncture block 230, and a Viterbi decoder block 232.

The variables V and U in beamforming blocks 212 and 218 respectivelyrefer to matrices utilized in the beamforming technique. U.S.application Ser. No. 11/052,389 filed Feb. 7, 2005, provides a detaileddescription of Eigen beamforming and is hereby incorporated herein byreference in its entirety.

The processor 240 may perform digital receiver and/or transmitterfunctions in accordance with applicable communications standards. Thesefunctions may comprise, but are not limited to; tasks performed at lowerlayers in a relevant protocol reference model. These tasks may furthercomprise the physical layer convergence procedure (PLCP), physicalmedium dependent (PMD) functions, and associated layer managementfunctions. The baseband processor 242 may similarly perform functions inaccordance with applicable communications standards. These functions maycomprise, but are not limited to, tasks related to analysis of datareceived via the receiver 201, and tasks related to generating data tobe transmitted via the transmitter 200. These tasks may further comprisemedium access control (MAC) layer functions as specified by pertinentstandards.

In the transmitter 200, the coding block 202 may transform receivedbinary input data blocks by applying a forward error correction (FEC)technique such as, for example, binary convolutional coding (BCC). Theapplication of FEC techniques, also known as “channel coding”, mayimprove the ability to successfully recover transmitted data at areceiver by appending redundant information to the input data prior totransmission via an RF channel. The ratio of the number of bits in thebinary input data block to the number of bits in the transformed datablock may be known as the “coding rate”. The coding rate may bespecified using the notation i_(b)/t_(b), where t_(b) represents thetotal number of bits that comprise a coding group of bits, while i_(b)represents the number of information bits that are contained in thegroup of bits t_(b). Any number of bits t_(b)-i_(b) may representredundant bits that may enable the receiver 201 to detect and correcterrors introduced during transmission. Increasing the number ofredundant bits may enable greater capabilities at the receiver to detectand correct errors in information bits. The penalty for this additionalerror detection and correction capability may result in a reduction inthe information transfer rates between the transmitter 200 and thereceiver 201. The invention is not limited to BCC and a plurality ofcoding techniques such as, for example, Turbo coding, or low densityparity check (LDPC) coding may also be utilized.

The puncture block 204 may receive transformed binary input data blocksfrom the coding block 202 and alter the coding rate by removingredundant bits from the received transformed binary input data blocks.For example, if the coding block 202 implemented a ½ coding rate, 4 bitsof data received from the coding block 202 may comprise 2 informationbits, and 2 redundant bits. By eliminating 1 of the redundant bits inthe group of 4 bits, the puncture block 204 may adapt the coding ratefrom ½ to ⅔. The interleaver block 206 may rearrange bits received in acoding rate-adapted data block from the puncture block 204 prior totransmission via an RF channel to reduce the probability ofuncorrectable corruption of data due to burst of errors, impactingcontiguous bits, during transmission via an RF channel. The output fromthe interleaver block 206, may also be divided into a plurality ofstreams where each stream may comprise a non-overlapping portion of thebits from the received coding rate-adapted data block. Therefore, for agiven number of bits in the coding rate-adapted data block, b_(db), agiven number of streams from the interleaver block 206, n_(st), and agiven number of bits assigned to an individual stream i by theinterleaver block 206, b_(st)(i): $\begin{matrix}{b_{db} = {\sum\limits_{i = 1}^{n_{st}}{b_{st}(i)}}} & {{equation}\quad\lbrack 5\rbrack}\end{matrix}$

The plurality of mapper blocks 208 a, . . . , 208 n may comprise anumber of individual mapper blocks that is equal to the number ofindividual streams generated by the interleaver block 206. Eachindividual mapper block 208 a, . . . , 208 n may receive a plurality ofbits from a corresponding individual stream, mapping those bits into a“symbol” by applying a modulation technique based on a “constellation”utilized to transform the plurality of bits into a signal levelrepresenting the symbol. The representation of the symbol may be acomplex quantity comprising in-phase (I) and quadrature (Q) components.The mapper block 208 a. . . 208 n for stream i may utilize a modulationtechnique to map a plurality of bits, b_(st)(i), into a symbol.

The beamforming V matrix block 212 may apply the beamforming techniqueto the plurality of symbols, or “spatial modes”, generated from theplurality of mapper blocks 208 a, . . . , 208 n. The beamforming Vmatrix block 212 may generate a plurality of signals where the number ofsignals generated may be equal to the number of transmitting antenna atthe transmitter 200. Each signal in the plurality of signals generatedby the beamforming V block 212 may comprise a weighted sum of at leastone of the received symbols from the mapper blocks 208 a, . . . , 208 n.

The plurality of IFFT blocks 210 a, . . . , 210 n may receive aplurality of signals from the beamforming block 212. Each IFFT block 210a, . . . , 210 n may subdivide the bandwidth of the RF channel into aplurality of n sub-band frequencies to implement orthogonal frequencydivision multiplexing (OFDM), buffering a plurality of received signals.Each buffered signal may be modulated by a carrier signal whosefrequency is based on of one of the sub-bands. Each of the IFFT blocks210 a, . . . , 210 n may then independently sum their respectivebuffered and modulated signals across the frequency sub-bands to performan n-point IFFT, thereby generating a composite OFDM signal.

The plurality of digital (D) to analog (A) conversion and antenna frontend blocks 214 a, . . . , 214 n may receive the plurality of signalsgenerated by the plurality of IFFT blocks 210 a, . . . , 210 n. Thedigital signal representation received from each of the plurality ofIFFT blocks 210 a, . . . , 210 n may be converted to an analog RF signalthat may be amplified and transmitted via an antenna. The plurality of Dto A conversion and antenna front end blocks 214 a, . . . , 214 n may beequal to the number of transmitting antenna 115 a, . . . , 115 n at thetransmitter 200. Each D to A conversion and antenna front end block 214a, . . . , 214 n may receive one of the plurality of signals from thebeamforming V matrix block 212 and may utilize an antenna 115 a, . . . ,115 n to transmit one RF signal via an RF channel.

In the receiver 201, the plurality antenna front end and A to Dconversion blocks 216 a, . . . , 216 n may receive analog RF signals viaan antenna, converting the RF signal to baseband and generating adigital equivalent of the received analog baseband signal. The digitalrepresentation may be a complex quantity comprising I and Q components.The number of antenna front end and A to D conversion blocks 216 a, . .. , 216 n may be equal to the number of receiving antenna 117 a, . . . ,117 n at the receiver 201.

The plurality of FFT blocks 220 a, . . . , 220 n may receive a pluralityof signals from the plurality of antenna front end and A to D conversionblocks 216 a, . . . , 216 n. The plurality of FFT blocks 220 a, . . . ,220 n may be equal to the number of antenna front end and A to Dconversion blocks 216 a, . . . , 216 n. Each FFT block 220 a, . . . ,220 n may receive a signal from an antenna front end and A to Dconversion block 216 a, . . . , 216 n, independently applying an n-pointFFT technique, demodulating the signal by a plurality of carrier signalsbased on the n sub-band frequencies utilized in the transmitter 200. Thedemodulated signals may be mathematically integrated over one sub bandfrequency period by each of the plurality of FFT blocks 220 a, . . . ,220 n to extract n symbols contained in each of the plurality of OFDMsignals received by the receiver 201.

The beamforming U* block 218 may apply the beamforming technique to theplurality of signals received from the plurality of FFT blocks 220 a, .. . , 220 n. The beamforming U* block 218 may generate a plurality ofsignals where the number of signals generated may be equal to the numberof streams utilized in generating the signals at the transmitter 200.Each of the plurality of signals generated by the beamforming U* block218 may comprise a weighted sum of at least one of the signals receivedfrom the FFT blocks 220 a, . . . , 220 n.

The channel estimates block 222 may utilize preamble informationcontained in a received RF signal to compute channel estimates. Theplurality of equalizer block 224 may receive signals generated by thebeamforming U* block 218. The equalizer block 224 may process thereceived signals based on input from the channel estimates block 222 torecover the symbol originally generated by the transmitter 200. Theequalizer block 224 may comprise suitable logic, circuitry, and/or codethat may be adapted to transform symbols received from the beamformingU* block to compensate for fading in the RF channel.

The plurality of demapper blocks 226 a . . . 226 n may receive symbolsfrom the plurality of equalizer blocks 224 a . . . 224 n, reversemapping each symbol to a plurality of bits by applying a demodulationtechnique, based on the modulation technique utilized in generating thesymbol at the transmitter 200, to transform the symbol into a pluralityof bits. The plurality of demapper blocks 226 a . . . 226 n may be equalto the number of equalizer blocks 224 a . . . 224 n, which may also beequal to the number of streams in the transmitter 200.

The deinterleaver block 228 may receive a plurality of bits from each ofthe demapper blocks 226 a . . . 226 n, rearranging the order of bitsamong the received plurality of bits. The deinterleaver block 228 mayrearrange the order of bits from the plurality of demapper blocks 226 a. . . 226 n in, for example, the reverse order of that utilized by theinterleaver 206 in the transmitter 200. The depuncture block 230 mayinsert “null” bits into the output data block received from thedeinterleaver block 228 that were removed by the puncture block 204. TheViterbi decoder block 232 may decode a depunctured output data block,applying a decoding technique that may recover the binary data blocksthat were input to the coding block 202.

In operation, the processor 240 may receive decoded data from theViterbi decoder 232. The processor 240 may communicate received data tothe baseband processor 242 for analysis and further processing. Theprocessor 240 may also communicate data received via the RF channel, bythe receiver 201, to the channel estimates block 222. This informationmay be utilized by the channel estimates block 222, in the receiver 201,to compute channel estimates for a received RF channel. The basebandprocessor 242 may generate data to be transmitted via an RF channel bythe transmitter 200. The baseband processor 242 may communicate the datato the processor 240. The processor 240 may generate a plurality of bitsthat are communicated to the coding block 202.

The elements shown in FIG. 2 b may comprise components that may bepresent in an exemplary embodiment of a wireless communicationsterminal. One exemplary embodiment of a may be a wireless communicationstransmitter comprising a transmitter 200, a processor 240, and abaseband processor 242. Another exemplary embodiment of a may be awireless communications receiver comprising a receiver 201, a processor240, and a baseband processor 242. Another exemplary embodiment of a maybe a wireless communications transceiver comprising a transmitter 200, areceiver 201, a processor 240, and a baseband processor 242.

Various embodiments of a MIMO system in an N_(TX)×N_(RX) antennaconfiguration may comprise a plurality of N_(TX) digital to analogconversion and antenna front end blocks 214 a . . . 214 n, and aplurality of N_(RX) antenna front end and analog to digital conversionblocks 216 a . . . 216 n.

FIG. 3 a illustrates an exemplary physical layer protocol data unit,which may be utilized in connection with an embodiment of the invention.With reference to FIG. 3 a there is shown a physical layer convergenceprotocol (PLCP) preamble field 302, a PLCP header field 304, and a datafield 306. The preamble field 302 may be utilized by a receiver 201 inconnection with the reception of signals via an RF channel. The headerfield 304 may comprise information that is utilized by a receiver 201 inconnection with the processing of information in the data field 306. Thedata field 306 may comprise information that is transmitted by atransmitter 200 and received by a receiver 201.

FIG. 3 b illustrates an exemplary data field in a PPDU, which may beutilized in connection with an embodiment of the invention. Withreference to FIG. 3 b there is shown a physical layer service data unit(PSDU) field 352, a tail field 354, and a pad field 356. In an exemplarydata field, as shown in FIG. 3 b, the PSDU may comprise a media accesscontrol (MAC) layer frame received from the MAC layer in the IEEE 802.11protocol stack. In an exemplary PPDU, as shown in FIG. 3 a, the datafield 306 may comprise 1,500 octets of binary data. The tail field 354may comprise a plurality of bits, the number of which may depend uponthe methods utilized to process the PSDU. The pad field 356 may comprisea plurality of bits, the number of which may depend upon a desirednumber of bits to be comprised in the data field.

FIG. 4 a shows exemplary training fields and header fields for mixedmode access in accordance with a TGn Sync proposal that may be utilizedin connection with an embodiment of the invention. With reference toFIG. 4 a there is shown a plurality of PPDU preambles and headers 402,422, and 442. The preamble and header 402 may comprise a legacy shorttraining field (L-STF) 404, a legacy long training field (L-LTF) 406, alegacy signal field (L-SIG) 408, a high throughput signal field (HT-SIG)410, a high throughput short training field for the first spatial stream(HT-STF₁) 412, and a plurality of high throughput long training fieldsfor the first spatial stream comprising training fields number 1 throughN (HT-LTF_(1,1) . . . HT-LTF_(1,N)) 414 . . . 416. The integer value Nmay represent the number of long training fields contained in thepreamble and header 402.

Each of the legacy short training fields, L-STF 404, 424, and 444 may beapproximately 8 μs in duration, or equivalent in time duration to 2 IEEE802.11n OFDM symbols and corresponding guard interval times, where eachsymbol and guard interval may be of approximately 4 μs in duration. Eachof the long training fields, L-LTF 406, 426, and 446 may beapproximately 8 μs in duration, or equivalent in time duration to 2 IEEE802.11n OFDM symbols and corresponding guard intervals. Each of thesignal fields L-SIG 408, 428, and 448 may be approximately 4 μs induration, or equivalent in time duration to 1 IEEE 802.11n OFDM symboland corresponding guard interval. Each of the high throughput HT-SIGfields 410, 430, and 450 may be approximately 8 μs in duration, orequivalent in time duration to 2 IEEE 802.11n OFDM symbols andcorresponding guard intervals. Each of the HT-STF fields 412, 432, and452 may be approximately 2.4 μs in duration. Each of the plurality ofHT-LTF_(1,1) . . . HT-LTF_(1,N), HT-LTF_(2,1) . . . HT-LTF_(2,N), . . ., HT-LTF_(NSS,1) . . . HT-LTF_(NSS,N) fields 414 . . . 416, 434 . . .436, . . . , 454 . . . 456 may be approximately 7.2 μs in duration.

The preamble and header 422 may comprise a legacy short training field424, a legacy long training field 426, a legacy signal field 428, a highthroughput signal field 430, a high throughput short training field forthe second spatial stream 432, and a plurality of high throughput longtraining fields for the second spatial stream comprising training fieldsnumber 1 through N 434 . . . 436. The preamble and header 442 maycomprise a legacy short training field 444, a legacy long training field446, a legacy signal field 448, a high throughput signal field 450, ahigh throughput short training field for spatial stream number N_(SS)452, and a plurality of high throughput long training fields for spatialstream number N_(SS) comprising training sequence fields number 1through N 454 . . . 456.

In operation, the integer value N_(SS) may represent the number ofspatial streams transmitted from a plurality of N_(TX) antennas locatedat a WLAN station. The number long training fields, N, may beapproximately equal to the number of spatial streams N_(SS). Thetraining field L-STF 424 may represent a time shifted version of thetraining field L-STF 404 based on a method such as cyclical diversitydelay (CDD). The training field L-STF 444 may represent a CDD version ofthe training field L-STF 424. The training field L-LTF 426 may representa CDD version of the training field L-LTF 406. The training field L-LTF446 may represent a CDD version of the training field L-LTF 426. Thesignal field L-SIG 428 may represent a CDD version of the signal fieldL-SIG 408. The signal field L-SIG 448 may represent a CDD version of thesignal field L-SIG 428. The signal field HT-SIG 430 may represent a CDDversion of the signal field HT-SIG 410. The signal field HT-SIG 450 mayrepresent a CDD version of the signal field HT-SIG 430.

The plurality of high throughput short training fields comprisingHT-STF₁ 412, HT-STF₂ 432, and HT-STF_(NSS) 452 may utilize toneinterleaving. In the tone interleaving procedure, a plurality of N_(TI)frequencies, or tones, from among the plurality of subcarrierfrequencies within a 20 MHz or 40 MHz RF channel, may be utilized fortransmission within a given training field, for example, the highthroughput short training field 412, 432, or 452 transmitted via each ofa plurality of N_(SS) spatial streams. Tones may be interleaved bydividing the plurality N_(TI) tones into a plurality of tone groups eachcomprising a plurality N_(TI)/N_(SS) tones such that no tone groupcomprises a tone whose frequency is approximately equal to the frequencyof a tone in another tone group. The HT-STF₁ may utilize tones from thefirst tone group, the HT-STF₂ may utilize tones from the second tonegroup, and so forth. Similarly, the plurality of long training fieldsHT-LTF_(1,1) 414, HT-LTF_(2,1) 434, and HT-LTF_(NSS,1) 454, may utilizetone interleaving. The plurality of long training fields HT-LTF_(1,N)416, HT-LTF_(2,N) 436, and HT-LTF_(NSS,N) 456, may utilize toneinterleaving.

FIG. 4 b shows an exemplary L-SIG header field for mixed mode access inaccordance with a TGn Sync proposal that may be utilized in connectionwith an embodiment of the invention. With reference to FIG. 4 b, thereis shown a L-SIG header 462. The L-SIG header 462 may comprise a ratefield 464, a reserve field 466, a length field 468, a parity field 470,and a tail field 472. The L-SIG header 462 may comprise 24 bits ofbinary information. The rate field 464 may comprise 4 bits of binaryinformation. The reserve field 466 may comprise 1 bit of binaryinformation. The length field 468 may comprise 12 bits of binaryinformation. The parity field 470 may comprise 1 bit of binaryinformation. The tail field 472 may comprise 6 bits of binaryinformation.

FIG. 4 c shows an exemplary HT-SIG header field for mixed mode access inaccordance with a TGn Sync proposal that may be utilized in connectionwith an embodiment of the invention. With reference to FIG. 4 c, thereis shown an HT-SIG header field 476. The HT-SIG header may comprise alength field 478, a modulation and coding scheme (MCS) field 480, anadvanced coding field 482, a reserved field 483, a sounding packet field484, a number of HT-LTF field 486, a short guard interval (GI) field488, an aggregation field 490, a scrambler initialization field 492, a20 MHz or 40 MHz bandwidth (BW) field 494, a cyclical redundancy checkfield 496, and a tail field 498. The length field 478 may comprise 18bits of binary information. The length field 478 may indicate the numberof octets of binary information that is contained in the physical layerservice data unit (PSDU) field 352 in the corresponding physical layerprotocol data unit (PPDU). The MCS field 480 may comprise 6 bits ofbinary information. The MCS field 480 may indicate the modulation typeand coding rate being utilized in the coding of the corresponding PPDU.The advanced coding field 482 may comprise 1 bit of binary information.The advanced coding field 482 may indicate whether binary convolutionalcoding (BCC), or low density parity check (LDPC) coding is utilized inthe coding of the corresponding PPDU. The reserved field 483 maycomprise 1 bit of binary information. The reserved field 483 maycomprise no assigned utilization.

The sounding packet field 484 may comprise, for example, 1 bit of binaryinformation. The sounding packet field 484 may indicate whether thecorresponding PSDU may be utilized for closed loop calibration between atransmitter and a receiver. The number of HT-LTF field 486 may comprise2 bits of binary information. The number of HT-LTF field 486 mayindicate the number of high throughput long training fields contained inthe corresponding PPDU. The short GI field 488 may comprise 1 bit ofbinary information. The short GI field 488 may indicate the length ofthe guard interval utilized when transmitting the data field 206 in thecorresponding PPDU. The aggregation field 490 may comprise 1 bit ofbinary information. The aggregation field 490 may indicate whether thedata field 306 in the corresponding PPDU comprises the last portion of amessage. The scrambler init field 492 may comprise 2 bits of binaryinformation. The scrambler init field 492 may be utilized to initializea scrambler function at the WLAN station receiving the PPDU. The 20 MHzor 40 MHz bandwidth field 494 may comprise 1 bit of binary information.The 20 MHz or 40 MHz bandwidth field 494 may indicate whether the PPDUwas transmitted utilizing a 20 MHz RF channel, or a 40 MHz RF channel.The CRC field 496 may comprise 8 bits of binary information. The CRCfield 496 may be utilized for detecting and/or correcting errors in areceived corresponding PPDU. The tail field 498 may comprise 6 bits ofbinary information. The tail field 498 may be utilized to extend thenumber of binary bits contained in an HT-SIG field to a desired length.

In an exemplary PPDU comprising 1,500 binary octets of data 306 (FIG. 3a), the data may comprise a time period of approximately 13 IEEE 802.11nOFDM symbols and corresponding guard bands in duration. This may bebased on transmitting at a data rate of 243 Mbits/s while utilizing 2spatial streams, 40 MHz bandwidth, 64 QAM modulation type, and a codingrate of ¾. Each OFDM symbol per spatial stream may comprise 486 bits ofbinary information for a combined 972 bits for the simultaneouslytransmitted OFDM symbols among the 2 spatial streams. The number ofbinary bits contained in an OFDM symbol, N_(DBPS), may be determinedbased on:N _(DBPS) =N _(DSC)*N_(B)(CON)*R, where   equation[6]N_(DSC), may represent the number of data bearing subcarriers in an RFchannel, N_(B)(CON) may represent the number of binary bits/symbol basedon the modulation type, and R may represent the coding rate. For a 40MHz RF channel, N_(DSC) may be approximately equal to 108. For amodulation type of 64 QAM, a symbol may comprise 6 binary bits. Thetotal number of bits simultaneously transmitted via N_(SS) number ofspatial streams may equal approximately N_(SS)×N_(DBPS).

Data 306 comprising 1,500 binary octets may comprise a time duration ofapproximately 13 OFDM symbols and corresponding guard bands. In the PPDUpreamble and header 402, legacy preamble, comprising the training fieldsL-STF 404, and L-LTF 406, and the signal field L-SIG 408, may comprise atime duration of approximately 5 OFDM symbols and corresponding guardbands. In the PPDU preamble and header 402, high throughput preamble andheader, comprising the signal field HT-SIG 412, and training fieldsL-LTF 414 and L-LTF 416, may comprise a time duration of approximately 6OFDM symbols and corresponding guard bands. The number of HT longtraining fields, N, may be equal to 2, for example.

A preamble and header 402, and data 306 comprising 1,500 binary octets,may produce a PPDU comprising a total time duration of approximately 24OFDM symbols and corresponding guard bands in duration. Given a timeduration of 4 μs for each OFDM symbol and corresponding guard band, thetotal time duration may be approximately 96 μs. Thus, the average datarate may be approximately equal to 1,500 binary octets, or 12,000 binarybits per 96 μs, or approximately 125 Mbits/s. Approximately 54% of thetotal duration may consist of data 306. Approximately 21% of the totalduration may comprise legacy preamble. Approximately 25% of the totalduration may comprise high throughput preamble and header.

Elimination of the legacy preamble from the preamble and header 402 mayenable an increase in data rate efficiency based on a data field 306comprising 1,500 binary octets. In this case, the average data rate maybe approximately 12,000 binary bits per 76 μs, or approximately 158Mbits/s, which is an increase of approximately 26% in the data rate.

If the modulation type were 64 QAM with the coding rate were decreasedfrom ¾ to ⅔, each OFDM symbol may comprise 432 binary bits information.In this case, data comprising 1,500 binary octets may comprise a timeduration of approximately 14 OFDM symbols and corresponding guard bands.Elimination of the legacy preamble from the preamble header 402 mayproduce an average data rate of approximately 12,000 binary bits per 80μs, or approximately 150 Mbits/s, which is an increase of approximately20% in the data rate.

If the modulation type were 64 QAM with the coding rate were decreasedfrom ¾ to ½, each OFDM symbol may comprise 424 binary bits information.In this case, data comprising 1,500 binary octets may comprise a timeduration of approximately 19 OFDM symbols and corresponding guard bands.Elimination of the legacy preamble from the preamble header 402 mayproduce an average data rate of approximately 12,000 binary bits per 100μs, or approximately 120 Mbits/s, which is a decrease of approximately4% in data rate.

A consequence of the ability to achieve coding rate reduction whilemaintaining comparable or higher data rates as a result of theelimination of legacy preamble from a PPDU may enable those comparabledata rates to be maintained at lower signal to noise ratios (SNR) thanmay be achievable when utilizing a PPDU preamble and header 402 thatcomprises legacy preamble. A coding rate reduction from ¾ to ⅔ due tothe elimination of legacy preamble resulting in greenfield access maycorrespond to at least 2 dB lower SNR for which a comparable, or higher,data rate may be maintained in comparison to a legacy preamble for mixedmode access PPDU with the preamble and header 402. This may be referredto as a greater than 2 dB performance gain for greenfield accessrelative to mixed mode access.

Utilization of LDPC coding to encode data 306 in a PPDU may produce aperformance gain relative to the use of BCC coding. The performance gainrealized from greenfield access may exceed that realized through theutilization of LDPC coding. Simulation results may show that LDPCprovides a performance gain of 2 dB relative to BCC utilized asspecified in IEEE 802.11a and in IEEE 802.11g. The utilization of LDPCmay add some complexity to embodiments of the receiver 201.

FIG. 5 a shows exemplary training fields and header fields forgreenfield access in accordance with a WWiSE proposal for N_(SS)=2, inaccordance with an embodiment of the invention. With reference to FIG. 5a, there is shown training fields and header for a first spatial stream502, and training fields and header for a second spatial stream 512. Thetraining fields and header 502 may comprise a high throughput (HT) shorttraining field for the first spatial stream (HT-ST₁) 504, a HT longtraining field for the first spatial stream (HT-LT₁) 506, and a Signal-Nfield for the first spatial stream (Signal-N₁) 508. The training fieldsand header 512 may comprise a HT short training field for the secondspatial stream (HT-ST₂) 514, a HT long training field for the secondspatial stream (HT-LT₂) 516, and a Signal-N field for the second spatialstream (Signal-N₂) 518.

In operation, a short training field may be utilized by a receiver for aplurality of reasons including, but not limited to, signal detection,automatic gain control (AGC) for low noise amplification circuitry,diversity selection performed by, for example, rake receiver circuitry,coarse frequency offset estimation, and timing synchronization. A longtraining field may be utilized by a receiver for a plurality of reasons,for example, fine frequency offset estimation, and channel estimation.The training field HT-ST₂ 514 may comprise a time shifted representationof the training field HT-ST₁ 504. The training field HT-LT₂ 516 maycomprise a time shifted representation of the training field HT-LT₁ 506.The signal field Signal-N₂ 518 may comprise a time shiftedrepresentation of the signal field Signal-N₁ 508. The training fields,HT-ST₁ 504 and HT-ST₂ 514, may comprise a time duration of about 8 μs,and further comprise a plurality of OFDM symbols, for example, aplurality of 10 ODFM symbols. The training fields, HT-LT₁ 506 and HT-LT₂516, may comprise a time duration of about 8 μs, and further comprise aplurality of OFDM symbols, for example, a plurality of 2 ODFM symbols.The signal fields, Signal-N₁ 508 and Signal-N₂ 518, may comprise a timeduration of about 4 μs, and further comprise an OFDM symbol.

FIG. 5 b shows an exemplary Signal-N header field for greenfield accessin accordance with a WWiSE proposal, in accordance with an embodiment ofthe invention. With reference to FIG. 5 b, there is shown a Signal-Nheader 552. The Signal-N header field may comprise a reserved field 554,a number of spatial streams (N_(SS)) field 556, a number of transmitantennas (NTX) field 558, a BW field 560, a coding rate (R) field 562,an error correcting code type (CT) field 564, a constellation type (CON)field 566, a length field 568, a last PSDU indicator (LPI) field 570, areserved field 572, a CRC field 574, and a tail field 576. The reservedfield 554 may comprise 6 bits of binary information. The reserved field572 may comprise 8 bits of binary information. The reserved fields 554and 572 may have no assigned usage. The N_(SS) field 556 may comprise 3bits of binary information. The N_(SS) field 556 may indicate the numberof spatial streams utilized in transmitting information from atransmitter, for example, transmitter 200, and a receiver, for examplereceiver 201. In a MIMO system, the number of spatial streams mayrepresent a number, for example, 1, 2, 3, or 4. The NTX field 558 maycomprise 3 bits of binary information. The NTX field 558 may indicatethe number of transmitting antenna utilized in transmitting informationbetween a transmitter and a receiver. In a MIMO system, the number oftransmitting antenna may represent a number, for example, 1, 2, 3, or 4.The BW field 560 may comprise 2 bits of binary information. The BW field560 may represent a bandwidth, for example, 20 MHz, or 40 MHz.

The R field 562 may comprise 3 bits of binary information. The R field628 may indicate the coding rate that is utilized for transmitting aphysical layer service data unit (PSDU) that is transmitted via anantenna. In a MIMO system, the coding rate may represent a number, forexample, ½, ⅔, ¾, or ⅚. The CT field 564 may comprise 2 bits of binaryinformation. The CT field 564 may indicate the error correcting code(ECC) type that is utilized in transmitting information via an antenna.In a MIMO system, the ECC type may represent an ECC method, for example,binary convolutional coding (BCC), or low density parity check coding(LDPC). The CON field 566 may comprise 3 bits of binary information. TheCON field 566 may indicate the constellation type, or modulation type,which is utilized in transmitting a PSDU via an antenna. In a MIMOsystem, the modulation type may represent a constellation indicating thenumber of binary bits that may be encoded in a symbol, for example,binary phase shift keying (BPSK), quaternary phase shift keying (QPSK),16 level quadrature amplitude modulation (16 QAM), 64 level QAM (64QAM), or 256 level QAM (256 QAM).

The length field 568 may comprise 13 bits of binary information. Thelength field 568 may comprise information that indicates the number ofbinary octets of data payload information, for example, the physicallayer service data unit (PSDU) 352. The LPI field 570 may comprise 1 bitof binary information. The LPI field 570 may comprise information thatindicates whether the corresponding PSDU 352 represents the lastinformation comprised in a message. The CRC field 574 may comprise 4bits of binary information. The CRC field 574 may comprise informationthat may be utilized by a receiver, for example, receiver 201, to detectthe presence of errors in a received PPDU. The tail field 576 maycomprise 6 bits of binary information. The tail field 576 may compriseinformation that is appended following the CRC field 574 to pad theSignal-N field to a desired length.

FIG. 5 c shows exemplary training fields and header fields forgreenfield access in accordance with a WWiSE proposal for N_(SS)=4, inaccordance with an embodiment of the invention. With reference to FIG. 5c, there is shown training fields and a header field for a first spatialstream 503, training fields and a header field for a second spatialstream 513, training fields and a header field for a third spatialstream 522, and training fields and a header field for a fourth spatialstream 532.

The training fields and header field for the first spatial stream 503may comprise a high throughput short training field HT-ST₁ 505, a firstHT long training field (HT-LT_(1,1)) 507, a Signal-N₁ field 508, and asecond HT long training field (HT-LT_(1,2)) 510. The training fields andheader field for the second spatial stream 513 may comprise a highthroughput short training field HT-ST₂ 515, a first HT long trainingfield (HT-LT_(2,1)) 517, a Signal-N₂ field 518, and a second HT longtraining field (HT-LT_(2,2)) 520. The training fields and header fieldfor the third spatial stream 522 may comprise a high throughput shorttraining field HT-ST₃ 524, a first HT long training field (HT-LT_(3,1))526, a Signal-N₃ field 528, and a second HT long training field(HT-LT_(1,2)) 530. The training fields and header field for the fourthspatial stream 532 may comprise a high throughput short training fieldHT-ST₄ 534, a first HT long training field (HT-LT_(4,1)) 536, aSignal-N₄ field 538, and a second HT long training field (HT-LT_(4,2))540.

In operation, the training field HT-ST₂ 515 may comprise a time shiftedrepresentation of the training field HT-ST₁ 505. The training fieldHT-ST₃ 524 may comprise a time shifted representation of the trainingfield HT-ST₂ 515. The training field HT-ST₄ 534 may comprise a timeshifted representation of the training field HT-ST₃ 524. The trainingfield HT-LT_(2,1) 517 may comprise a time shifted representation of thetraining field HT-LT_(1,1) 507. The training field HT-LT_(3,1) 526 maycomprise a time shifted representation of the training field HT-LT_(2,1)517. The training field HT-LT_(4,1) 536 may comprise a time shiftedrepresentation of the training field HT-LT_(3,1) 526. The signal fieldSignal-N₂ 518 may comprise a time shifted representation of the signalfield Signal-N₁ 508. The signal field Signal-N₃ 528 may comprise a timeshifted representation of the signal field Signal-N₂ 518. The signalfield Signal-N₄ 538 may comprise a time shifted representation of thesignal field Signal-N₃ 528. The training field HT-LT_(2,2) 520 maycomprise a time shifted representation of the training field HT-LT_(1,2)510. The training field HT-LT_(3,2) 530 may comprise a time shiftedrepresentation of the training field HT-LT_(2,2) 520. The training fieldHT-LT_(4,2) 540 may comprise a time shifted representation of thetraining field HT-LT_(3,2) 530.

The training fields, HT-ST₁ 505, HT-ST₂ 515, HT-ST₃ 524, and HT-ST₄ 534,may comprise a time duration of about 8 μs, and further comprise aplurality of OFDM symbols, for example, a plurality of 10 ODFM symbols.The training fields, HT-LT_(1,1) 507, HT-LT_(1,2) 510, HT-LT_(2,1) 517,HT-LT_(2,2) 520, HT-LT_(3,1) 526, HT-LT_(3,2) 530, HT-LT_(4,1) 536, andHT-LT_(4,2) 540, may comprise a time duration of about 8 μs, and furthercomprise a plurality of OFDM symbols, for example, a plurality of 2 ODFMsymbols. The signal fields, Signal-N₁ 508 Signal-N₂ 518, Signal-N₃ 528and Signal-N₄ 538, may comprise a time duration of about 4 μs, andfurther comprise an OFDM symbol.

Comparing FIG. 5 a and FIG. 5 c, the exemplary training fields andSignal-N header field illustrated in FIG. 5 a, based on 2 spatialstreams, 502 and 512, may each be of approximately 20 μs in duration, orequivalent in time duration to 5 IEEE 802.11n OFDM symbols andcorresponding guard bands. The exemplary training fields and Signal-Nheader field illustrated in FIG. 5 c, based 4 spatial streams, 503, 513,522, and 532, may each be of approximately 28 μs in duration, orequivalent in time duration to 7 IEEE 802.11n OFDM symbols andcorresponding guard bands.

FIG. 6 a shows exemplary training fields and header fields with trailingsignal field for greenfield access for N_(SS)>2, in accordance with anembodiment of the invention. With reference to FIG. 6 a there is showntraining fields and a header field for a first spatial stream 602,training fields and a header field for a second spatial stream 622, andtraining fields and a header field for spatial stream N_(SS) 642. Thetraining fields and header field for the first spatial stream 602 maycomprise a short training field HT-STF₁ field 604, a long training fieldHT-LTF_(1,1) field 606, a plurality of subsequent long training fieldsHT-LTF_(1,2) . . . HT-LTF_(1,N) 608 . . . 610, and a Signal*-N₁ field612. The training sequence and header field for the second spatialstream 622 may comprise an HT-STF₂ field 624, an HT-LTF_(2,1) field 626,a plurality of HT-LTF_(2,2) . . . HT-LTF_(2,N) fields 628 . . . 630, anda Signal*-N₂ field 632. The training sequence and header field forspatial stream N_(SS) 642 may comprise an HT-STF_(NSS) field 644, anHT-LTF_(NSS,1) field 646, a plurality of HT-LTF_(NSS,2) . . .HT-LTF_(NSS,N) fields 648 . . . 650, and a Signal*-N_(NSS) field 652.The Signal*-N fields 612, 632, and 652 may be represented as shown inFIG. 4 c.

In operation, the short training sequence utilized in the training fieldHT-STF₁ 604, STS₁, may be represented as a vector comprising a pluralityof coefficients. The short training sequence utilized in the trainingfield HT-STF₂ 624, STS₂, may be represented as a vector comprising aplurality of coefficients. The short training sequence utilized in thetraining field HT-STF_(NSS) 624, STS_(NSS), may be represented as avector comprising a plurality of coefficients. Each vectorrepresentation among the plurality of vector representations STS₁ . . .STS_(NSS) may be orthonormal to each of the other vector representationsin the plurality of vector representations.

The long training sequence utilized in the first training field of thefirst spatial stream, HT-LTF_(1,1) 606, LTS_(1,1), may be represented asa vector comprising a plurality of coefficients. The long trainingsequence utilized in the first training field of the second spatialstream HT-LTF_(2,1) 626, LTS_(2,1), may be represented as a vectorcomprising a plurality of coefficients. The long training sequenceutilized in the first training field of spatial stream N_(SS)HT-LTF_(NSS,1) 646, LTS_(NSS,1), may be represented as a vectorcomprising a plurality of coefficients. Each vector representation amongthe plurality of vector representations LTS_(1,1) . . . LTS_(NSS,1) maybe orthonormal to each of the other vector representations in theplurality of vector representations.

The long training sequence utilized in the second training field of thefirst spatial stream HT-LTF_(1,2) 608, LTS_(1,2), may be represented asa vector comprising a plurality of coefficients. The long trainingsequence utilized in the second training field of the second spatialstream HT-LTF_(2,2) 628, LTS_(2,2,) may be represented as a vectorcomprising a plurality of coefficients. The long training sequenceutilized in the second training field of spatial stream N_(SS)HT-LTF_(NSS,2) 648, LTS_(NSS,2), may be represented as a vectorcomprising a plurality of coefficients. Each vector representation amongthe plurality of vector representations LTS_(1,2) . . . LTS_(NSS,2) maybe orthonormal to each of the other vector representations in theplurality of vector representations.

The long training sequence utilized in the training field N of the firstspatial stream HT-LTF_(1,N) 610, LTS_(1,N), may be represented as avector comprising a plurality of coefficients. The long trainingsequence utilized in the training field N of the second spatial streamHT-LTF_(2,N) 630, LTS_(2,N,) may be represented as a vector comprising aplurality of coefficients. The long training sequence utilized in thetraining field N of spatial stream N_(SS) HT-LTF_(NSS,N) 650,LTS_(NSS,N,) may be represented as a vector comprising a plurality ofcoefficients. Each vector representation among the plurality of vectorrepresentations LTS_(1,N) . . . LTS_(NSS,N) may be orthonormal to eachof the other vector representations in the plurality of vectorrepresentations. The number of long training fields, N, may beapproximately equal to the number of spatial streams, N_(SS).

Orthonormality is a property of vectors such that for any two vectors, Xand Y, the vector dot product of the vectors may equal zero. Whenapplied to long training sequences, the property of orthonormality mayresult in the generation of long training sequences whose vectorrepresentations exhibit the property of orthonormality. The generationof an orthonormal long training sequence may produce phase shifts amongthe frequency subcarriers that comprise an OFDM symbol generated basedon the long training sequence. The phase shifts may improve the qualityof transmitted OFDM symbols by reducing the likelihood of accidentalnulls in the beam pattern of the signals transmitted by a transmitter200 (FIG. 2 b). The utilization of known phase shifts among thefrequency subcarriers may enable a receiver 201 to remove the phaseshifts in a received signal during channel estimation. The placement ofthe Signal*-N fields 612, 632, and 652 following the correspondingpluralities of long training sequence fields in each of the spatialstreams may enable a receiver to utilize a full channel estimate, basedon the preceding long training fields for each spatial stream, forexample, long training sequence fields 606 and 608 . . . 610 in thefirst spatial stream, in detecting the corresponding Signal*-N field.

In MIMO systems, orthonormal sequences may enable a receiver 201(FIG. 1) to more easily distinguish a signal transmitted from a specifictransmitter antenna front end 214 a . . . 214 n at a transmitter 200. Amatched filter at receiver antenna front ends 216 a . . . 216 n at thereceiver 201 may enable the receiver to receive a signal transmitted bya specific transmitter antenna front end 214 a . . . 214 n at a specificreceiver antenna front end 216 a . . . 216 n.

The training fields, HT-STF₁ 604, HT-STF₂ 624, and HT-STF_(NSS) 644, maycomprise a time duration of about 8 μs, and further comprise a pluralityof OFDM symbols, for example, a plurality of 10 ODFM symbols. Thetraining fields, HT-LTF_(1,1) 606, HT-LTF_(2,1) 626, and HT-LTF_(NSS,1)646, may comprise a time duration of about 8 μs, and further comprise aplurality of OFDM symbols, for example, a plurality of 2 ODFM symbols.The plurality of OFDM symbols in training field 606 may be identical.The plurality of OFDM symbols in training field 626 may be identical.The plurality of OFDM symbols in training field 646 may be identical.The pluralities of training fields, HT-LTF_(1,2) . . . HT-LTF_(1,N) 608. . . 610, HT-LTF_(2,2) . . . HT-LTF_(2,N) 628 . . . 630, andHT-LTF_(NSS,2) . . . HT-LTF_(NSS,N) 648 . . . 650, may comprise a timeduration of about 4 μs, and further comprise an OFDM symbol. Utilizingorthonormal training sequences after the first long training sequencemay obviate tone interleaving, which may be a desirable feature becausethe first long training sequence may utilize identical symbols. Thesignal fields, Signal*-N 612, Signal*-N₂ 632, and Signal*-N_(NSS) 652,may comprise a time duration of about 8 μs, and further comprise aplurality of OFDM symbols, for example, a plurality of 2 OFDM symbols.

For N_(SS)=2 there may be a plurality of N=2 long training fields inexemplary training fields and header field 602, 622, or 642. Referringto FIG. 6 a for the case of 2 transmitted spatial streams, the trainingfields and Signal*-N header field 602, 622, and 642 may comprise a timeduration of about 28 μs, and further comprise a plurality of 7 IEEE802.11n OFDM symbols. For the case of 3 transmitted spatial streams, thetraining fields and header field 602, 622, and 642 may comprise a timeduration of about 32 μs, and further comprise a plurality of 8 IEEE802.11n OFDM symbols. For the case of 4 transmitted spatial streams, thetraining fields and header field 602, 622, and 642 may comprise a timeduration of about 36 μs, and further comprise a plurality of 9 IEEE802.11n OFDM symbols.

Comparing the training fields and header field 402, 422, or 442 (FIG. 4a) for mixed mode access in an IEEE 802.11n WLAN to comparable trainingfields and header field 602, 622, or 642 (FIG. 6 a) for greenfieldaccess in an IEEE 802.11n WLAN for the case of 2 transmitted spatialstreams may indicate that the training fields and header field 602, 622,or 642 may comprise a time duration that is approximately 16 μs shorterin duration than that of comparable training fields and header field402, 422, or 442. This may correspond to a reduction of 4 fewer IEEE802.11n OFDM symbols transmitted with each physical layer protocol dataunit (PPDU).

In various embodiments of the invention, as illustrated in the exemplarytraining fields and header field in FIG. 6 a, the Signal*-N field berepresented as described in FIG. 4 c. The Signal*-N field may comprise atime duration of approximately 8 μs, and further comprise 2 OFDMsymbols. Each of the first high throughput long training fields amongthe spatial streams, HT-LTF_(1,1) 606, HT-LTF_(2,1) 626, andHT-LTF_(NSS,1) 646 may comprise a time duration of approximately 8 μs,and further comprise 2 OFDM symbols. Each of the first high throughputlong training fields among the spatial streams, HT-LTF_(1,1) 606,HT-LTF_(2,1) 626, and HT-LTF_(NSS,1) 646 may comprise identical OFDMsymbols that may be utilized for fine frequency offset estimation as maybe specified in IEEE resolutions 802.11a, and 802.11g.

FIG. 6 b shows exemplary training fields and header fields with earlysignal field for greenfield access for N_(SS)>2, in accordance with anembodiment of the invention. FIG. 6 b differs from FIG. 6 a in that, inFIG. 6 b, a signal field may follow a first long training field in aspatial stream PDU, with one or more subsequent long training fieldsfollowing the signal field. With reference to FIG. 6 b there is showntraining fields and a header field for a first spatial stream 602 a,training fields and a header field for a second spatial stream 622 a,and training fields and a header field for spatial stream N_(SS) 642 a.The training fields and header field for the first spatial stream 602 amay comprise a short training field HT-STF₁ field 604 a, a long trainingfield HT-LTF_(1,1) field 606 a, a plurality of subsequent long trainingfields HT-LTF_(1,2) . . . HT-LTF_(1,N) 608 a . . . 610 a, and aSignal*-N, field 612 a. The training sequence and header field for thesecond spatial stream 622 a may comprise an HT-STF₂ field 624 a, anHT-LTF_(2,1) field 626 a, a plurality of HT-LTF_(2,2) . . . HT-LTF_(2,N)fields 628 a . . . 630 a, and a Signal*-N₂ field 632 a. The trainingsequence and header field for spatial stream N_(SS) 642 a may comprisean HT-STF_(NSS) field 644 a, an HT-LTF_(NSS,1) field 646 a, a pluralityof HT-LTF_(NSS,2) . . . HT-LTF_(NSS,N) fields 648 a . . . 650 a, and aSignal*-N_(NSS) field 652 a.

Long training sequences may be utilized for generating OFDM symbols thatmay be transmitted during long training fields. Long training sequencesfor N_(SS)=2 may be defined as follows: $\begin{matrix}{\begin{matrix}{{HT} -} \\{{LTF}\quad} \\\lbrack {i\text{,}j} \rbrack\end{matrix} = \begin{bmatrix}{{.11}{aLT}} & {{.11}{aLT}} \\{{- {.11}}{aLT}*{\mathbb{e}}^{j*{theta}\quad{(k)}}} & {{.11}{aLT}*{\mathbb{e}}^{j*{theta}\quad{(k)}}}\end{bmatrix}} & {{equation}\quad\lbrack 7\rbrack}\end{matrix}$where the index, i, may represent a row in the matrix, and the index, j,may represent a column. Each row may represent a corresponding spatialstream, with each column representing a corresponding long trainingsequence, .11a LT indicates that the training sequence may be based onspecifications in IEEE 802.11a, theta(k) may indicate a phase shift inthe LT field for OFDM subcarrier k in an RF channel where the phaseshift may vary as a function of the index k.

Individual elements in the long training sequence, .11aLT, based on IEEE802.11a for a 20 MHz channel, may be represented utilizing the vectornotation, LS[k], where k may comprise a range of integer values from andincluding −N_(SC)/2, up to and including N_(SC)/2. as: $\begin{matrix}{{{.11}{aLT}} = \begin{Bmatrix}{1,1,1,1,{- 1},{- 1},1,1,{- 1},1,1,1,} \\{1,1,1,{- 1},{- 1},1,1,{- 1},1,{- 1},1,} \\{1,1,1,0,1,{- 1},{- 1},1,1,{- 1},1,} \\{{- 1},1,{- 1},{- 1},{- 1},{- 1},{- 1},1,1,{- 1},} \\{{- 1},1,{- 1},1,{- 1},1,1,1,1,{- 1},{- 1}}\end{Bmatrix}} & {{equation}\quad\lbrack 8\rbrack}\end{matrix}$where the first element in equation[8] may represent LS[−N_(SC)/2] andthe last element may represent LS[N_(SC)/2].

A long OFDM training symbol, r_(LONG)(t), where the variable t mayrepresent time, may be generated according to the equation:$\begin{matrix}{{r_{LONG}(t)} = {{w_{TLONG}(t)}{\sum\limits_{k = {N_{sc}/2}}^{N_{sc}/2}{{{LS}\quad\lbrack k\rbrack}\quad{\mathbb{e}}^{{j2\pi 2\Delta}_{F}{({t - T_{G12}})}}}}}} & {{equation}\quad\lbrack 9\rbrack}\end{matrix}$where N_(SC) may represent the number of frequency subcarriers, Δ_(f),may represent the frequency spacing between subcarriers, T_(GI2) mayrepresent the training symbol guard band time interval, and w_(TLONG)(t)may represent the timing window for the long training sequence. Thetiming window w_(TLONG)(t) may be represented as: $\begin{matrix}{{{w_{T}( {nT}_{s} )} = \begin{Bmatrix}1 & {1 \leq n \leq 79} \\0.5 & {0,80} \\0 & {otherwise}\end{Bmatrix}},{where}} & {{equation}\quad\lbrack 10\rbrack}\end{matrix}$the sampling interval, T_(S), may equal approximately 50 ns for a 20 MHzchannel, and n may represent a sample of the OFDM signal as representedby equation[9] taken at a time t=nT_(S) during a timing window intervalfor transmission of an OFDM symbol.

With reference to equation[9] for N_(SS)=2, substitution of a longtraining sequence element HT-LTF[i,j] from equation[7] for LS[k] inequation[9] may be utilized to generate an OFDM symbol for the j^(th)long training field in the i^(th) spatial stream. In the first longtraining field, the OFDM symbol generated by equation[9] may betransmitted twice.

The LT fields may be defined for N_(SS)=3 as follows: $\begin{matrix}{{{HT} - {{LTF}\quad\lbrack {i,j} \rbrack}} = \lbrack \quad \quad\begin{matrix}{{.11}{aLT}*W\quad 11} & {{.11}{aLT}*W\quad 12} & {{.11}{aLT}*W\quad 13} \\{{.11}{aLT}*W\quad 21*{\mathbb{e}}^{j*{theta}\quad{(k)}}} & {{.11}{aLT}*W\quad 22*{\mathbb{e}}^{j*{theta}\quad{(k)}}} & {{.11}{aLT}*W\quad 22*{\mathbb{e}}^{j*{theta}\quad{(k)}}} \\{{.11}{aLT}*W\quad 31*{\mathbb{e}}^{j*{phi}\quad{(k)}}} & {{.11}{aLT}*W\quad 32*{\mathbb{e}}^{j*{phi}\quad{(k)}}} & {{.11}{aLT}*W\quad 33*{\mathbb{e}}^{j*{phi}\quad{(k)}}}\end{matrix}\quad \rbrack } & {{equation}\quad\lbrack 11\rbrack}\end{matrix}$where the index, i, may represent a row in the matrix, and the index, j,may represent a column. Each row may represent a corresponding spatialstream, with each column representing a corresponding long trainingsequence. W_(mn) may represent elements from a discrete Fouriertransform (DFT) matrix, and phi(k) indicates a phase shift in the LTfield for OFDM subcarrier k where the phase shift varies as a functionof k where phi(k) may not equal theta(k).

The DFT matrix, W_(mn), may be represented as: $\begin{matrix}{{W_{mn} = \begin{bmatrix}1 & 1 & 1 \\1 & \frac{{- 1} - {i\sqrt{3}}}{2} & \frac{{- 1} + {i\sqrt{3}}}{2} \\1 & \frac{{- 1} + {i\sqrt{3}}}{2} & \frac{{- 1} - {i\sqrt{3}}}{2}\end{bmatrix}},{where}} & {{equation}\quad\lbrack 12\rbrack}\end{matrix}$the index, m, may represent a row in the matrix, and the index, n, mayindicate a column.

With reference to equation[9] for N_(SS)=3, substitution of a longtraining sequence element HT-LTF[i,j] from equation[11] for LS[k] inequation[9] may be utilized to generate an OFDM symbol for the j^(th)long training field in the i^(th) spatial stream. In the first longtraining field, the OFDM symbol generated by equation[9] may betransmitted twice.

The LT fields may be defined for N=4 as follows: $\begin{matrix}{\begin{matrix}{{HT} - {LFT}} \\\lbrack {i,j} \rbrack\end{matrix} = \lbrack \quad\begin{matrix}{{- 1}*{.11}{aLT}} & {{.11}{aLT}} & {{.11}{aLT}} & {{.11}{aLT}} \\{{.11}{aLT}*{\mathbb{e}}^{j*{theta}\quad{(k)}}} & {{- 1}*{.11}{aLT}*{\mathbb{e}}^{j*{theta}\quad{(k)}}} & {{.11}{aLT}*{\mathbb{e}}^{j*{theta}\quad{(k)}}} & {{.11}{aLT}*{\mathbb{e}}^{j*{theta}\quad{(k)}}} \\{{.11}{aLT}*{\mathbb{e}}^{j*{phi}\quad{(k)}}} & {{.11}{aLT}*{\mathbb{e}}^{j*{phi}\quad{(k)}}} & {{- 1}*{.11}{aLT}*{\mathbb{e}}^{j*{phi}\quad{(k)}}} & {{.11}{aLT}*{\mathbb{e}}^{j*{phi}\quad{(k)}}} \\{{.11}{aLT}*{\mathbb{e}}^{j*{psi}\quad{(k)}}} & {{.11}{aLT}*{\mathbb{e}}^{j*{psi}\quad{(k)}}} & {{.11}{aLT}*{\mathbb{e}}^{j*{psi}\quad{(k)}}} & {{- 1}*{.11}{aLT}*{\mathbb{e}}^{j*{psi}\quad{(k)}}}\end{matrix}\quad \rbrack} & {{equation}\quad\lbrack 13\rbrack}\end{matrix}$where the index, i, may represent a row in the matrix, and the index, j,may represent a column. Each row may represent a corresponding spatialstream, with each column representing a corresponding long trainingsequence. phi(k) indicates a phase shift in the LT field for OFDMsubcarrier k where the phase shift varies as a function of k. psi(k)indicates a phase shift in the LT field for OFDM subcarrier k where thephase shift varies as a function of k. The phase shifts phi(k),theta(k), and psi(k) may not be equal.

With reference to equation[9] for N_(SS)=4, substitution of a longtraining sequence element HT-LTF[i,j] from equation[13] for LS[k] inequation[9] may be utilized to generate an OFDM symbol for the j^(th)long training field in the i^(th) spatial stream. In the first longtraining field, the OFDM symbol generated by equation[9] may betransmitted twice.

The long training sequence as represented in equation[8] may be utilizedto generate a plurality of orthonormal long training sequences.Alternatively, orthonormal long training sequence fields may begenerated. If the long training sequences in different long trainingsequence fields are orthonormal, the corresponding long trainingsequence fields may also be orthonormal. Orthonormal long trainingsequences or long training sequence fields may be generated by aplurality of methods. In one embodiment of the invention, orthonormallong training sequence fields may be generated by utilizing a discreteFourier transform matrix to apply phase shifts to individual longtraining sequence fields among a plurality of long training sequencefields in a spatial stream. The utilization of a discrete Fouriertransform matrix may enable the orthonormal generator sequence to be ofminimum length. The property of orthonormality may be observed in that along training sequence field in a current spatial stream may beorthogonal to a corresponding long training sequence field in asubsequent spatial stream. A plurality of orthogonal long trainingsequences fields, HT-LTF_(n)[i,j], for a plurality of spatial streamsmay be generated according to the following relationship:HT-LTF _(n) [i,j]=HT-LTF[i,j]e ^(−j2πij/N) ^(SS) , where   equation[14]the index i may refer to an individual spatial stream among a pluralityof spatial streams, the index j may refer to an individual long trainingsequence field within a spatial stream, N_(SS) may refer to the numberof transmitted spatial streams, and HT-LTF[i,j] may refer to anindividual long training sequence field as represented in any ofequations[7], [11], or [13]. As an example of the property oforthonormality, a j^(th) high throughput long training sequence fieldfor a first spatial stream HT-LTF_(n)[1,j] may be orthonormal to acorresponding j^(th) high throughput long training sequence field for asecond spatial stream HT-LTF_(n)2,j].

In another embodiment of the invention, a discrete Hadamard transformmay be utilized to generate orthonormal long training sequences based ona Hadamard matrix. A property of a Hadamard matrix is that the matrixmay comprise values of +1 and −1 such that the rows of the Hadamardmatrix may be mutually orthogonal. For example, a long trainingsequence, O(.11aLT), that may be orthonormal to the long trainingsequence .11aLT as expressed in equation[8] is: $\begin{matrix}{{{.11}{aLT}} = \begin{Bmatrix}{1,{- 1},1,{- 1},{- 1},1,1,{- 1},{- 1},{- 1},} \\{{- 1},{- 1},1,{- 1},1,{- 1},1,1,{- 1},{- 1},1,} \\{1,1,1,1,{- 1},1,{- 1},0,1,1,{- 1},{- 1},} \\{1,1,1,1,1,1,{- 1},1,{- 1},1,1,{- 1},} \\{{- 1},1,1,1,1,1,1,{- 1},1,{- 1},{- 1},1}\end{Bmatrix}} & {{equation}\quad\lbrack 15\rbrack}\end{matrix}$

Various embodiments of the invention may not be limited in the methodsthat may be utilized in generating orthonormal long training sequencesor orthonormal long training sequence fields.

FIG. 7 shows exemplary training fields and header fields for mixed modeaccess for N_(SS)>2, in accordance with an embodiment of the invention.With reference to FIG. 7 there is shown a plurality of PPDU preamblesand headers 702, 722, and 742. The preamble and header 702 may comprisea legacy short training field (L-STF) 404, a legacy long training field(L-LTF) 406, a legacy signal field (L-SIG) 408, a high throughput signalfield (HT-SIG) 410, a high throughput short training field for the firstspatial stream (HT-STF₁) 712, and a plurality of high throughput longtraining fields for the first spatial stream comprising training fieldsnumber 1 through N (HT-LTF_(1,1) . . . HT-LTF_(1,N)) 714 . . . 716. Theinteger value N may represent the number of long training fieldscontained in the preamble and header 702.

The preamble and header 722 may comprise a legacy short training field(L-STF) 424, a legacy long training field (L-LTF) 426, a legacy signalfield (L-SIG) 428, a high throughput signal field (HT-SIG) 430, a highthroughput short training field for the second spatial stream (HT-STF₂)732, and a plurality of high throughput long training fields for thesecond spatial stream comprising training fields number 1 through N(HT-LTF_(2,1) . . . HT-LTF_(2,N)) 734 . . . 736. The integer value N mayrepresent the number of long training fields contained in the preambleand header 722.

The preamble and header 742 may comprise a legacy short training field(L-STF) 444, a legacy long training field (L-LTF) 446, a legacy signalfield (L-SIG) 448, a high throughput signal field (HT-SIG) 450, a highthroughput short training field for the spatial stream N_(SS)(HT-STF_(NSS)) 752, and a plurality of high throughput long trainingfields for the spatial stream N_(SS) comprising training fields number 1through N (HT-LTF_(NSS,1) . . . HT-LTF_(NSS,N)) 754 . . . 756. Theinteger value N may represent the number of long training fieldscontained in the preamble and header 702.

The HT-STF fields 712, 732, and 752 may each comprise a time duration ofabout 3.2 μs, and the HT-LTF fields 714 . . . 716, 734 . . . 736, and754 . . . 756 may each comprise a time duration of about 4 μs. The timeduration of about 3.2 μs for the HT-STF fields 712, 732, and 752 mayrepresent an increase of about 800 ns in time duration relative to thecorresponding time duration of about 2.4 μs for the HT-STF fields 412,432, and 452. The increase of about 800 ns in time duration may allowmore time for automatic gain control settling in adapting transmissionof signals to utilize beamforming.

Comparing the training fields and header fields 702, 722, and 742 to thecomparable the training fields and header fields 402, 422, and 442 forthe case of 2 transmitted spatial streams, wherein N=2, may indicatethat the training fields and header fields 702, 722, and 742 maycomprise a time duration about 5.6 μs shorter than for the comparabletraining fields and header fields 402, 422, and 442. Comparing thetraining fields and header fields 702, 722, and 742 to the comparablethe training fields and header fields 402, 422, and 442 for the case of3 transmitted spatial streams, wherein N=3, may indicate that thetraining fields and header fields 702, 722, and 742 may comprise a timeduration of about 8.8 μs shorter than for the comparable training fieldsand header fields 402, 422, and 442. Comparing the training fields andheader fields 702, 722, and 742 to the comparable the training fieldsand header fields 402, 422, and 442 for the case of 4 transmittedspatial streams, wherein N=2, may indicate that the training fields andheader fields 702, 722, and 742 may comprise a time duration about 12 μsshorter than for the comparable training fields and header fields 402,422, and 442.

Various embodiments of the invention may provide a system forcommunicating information in a multiple input multiple output (MIMO)communications system that may comprise a transmitter 200 (FIG. 2 b)that generates a protocol data unit (PDU) for a current spatial streamcomprising a current plurality of long training sequence fields. Thetransmitter 200 may generate a PDU for a subsequent spatial streamcomprising a subsequent plurality of long training sequence fieldswherein one of the subsequent plurality of long training sequence fieldsis orthonormal to a corresponding one of the current plurality of longtraining sequence fields. The transmitter 200 may append a signal fieldsubsequent to the last of the plurality of long training sequencefields.

Various embodiments of the invention may provide a system forcommunicating information in a multiple input multiple output (MIMO)communications system that may comprise a transmitter that constructs agreenfield protocol data unit (PDU) comprising a high throughput shorttraining sequence field comprising a time duration of approximately 8μs. The transmitter may append a first long training sequence field,comprising a time duration of approximately 8 μs, subsequent to thethigh throughput short training sequence field. The transmitter may alsoappend at least one subsequent long training sequence field, comprisinga time duration of approximately 4 μs, subsequent to the first longtraining sequence field. In addition, the transmitter may append asignal field, comprising a time duration of approximately 8 μs,subsequent to the last of at least one subsequent long training sequencefield.

Aspects of a method for communicating information in a multiple inputmultiple output (MIMO) communications system that may compriseconstructing a greenfield protocol data unit (PDU) comprising a highthroughput short training sequence field comprising a time duration ofapproximately 8 μs. The method may further comprise appending a firstlong training sequence field, comprising a time duration ofapproximately 8 μs, subsequent to the high throughput short trainingsequence field. At least one subsequent long training sequence field,comprising a time duration of approximately 4 μs, may be appendedsubsequent to the first long training sequence field. A signal field,comprising a time duration of approximately 8 μs, may also be appendedsubsequent to the last of at least one subsequent long training sequencefield.

Aspects of a method for communicating information in a multiple inputmultiple output (MIMO) communications system may comprise constructing amixed mode protocol data unit (PDU) comprising a legacy short trainingsequence field comprising a time duration of approximately 8 μs. Alegacy long training sequence field comprising a time duration ofapproximately 8 μs may be appended. A legacy signal field comprising atime duration of approximately 4 μs may be appended. The method maycomprise appending a high throughput signal field, comprising a timeduration of approximately 8 μs, subsequent to the legacy signal field.The method may further comprise appending a high throughput shorttraining sequence field comprising a time duration of approximately 3.2μs, subsequent to the high throughput signal field, and subsequentlyappending a plurality of long training sequence fields, comprising atime duration of approximately 4 μs.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputer system, or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computer system with a computerprogram that, when being loaded and executed, controls the computersystem such that it carries out the methods described herein.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

1. A method for communicating information in a multiple input multipleoutput (MIMO) communications systems, the method comprising: generatinga protocol data unit (PDU) for a current spatial stream comprising acurrent plurality of long training sequence fields; appending a signalfield subsequent to a last of said current plurality of long trainingsequence fields; generating a PDU for a subsequent spatial streamcomprising a subsequent plurality of long training sequence fieldswherein one of said subsequent plurality of long training sequencefields is orthonormal to a corresponding one of said current pluralityof long training sequence fields; and appending a subsequent signalfield subsequent to a last of said subsequent plurality of long trainingsequence fields.
 2. The method according to claim 1, further comprisingtransmitting 2 orthogonal frequency division multiplexing (OFDM) symbolsduring a first long training sequence field of said current plurality oflong training sequence fields.
 3. The method according to claim 2,further comprising generating said 2 OFDM symbols based on long trainingsequences that are approximately identical.
 4. The method according toclaim 2, further comprising transmitting 1 OFDM symbol during a longtraining sequence field that is subsequent to said first long trainingsequence field.
 5. The method according to claim 1, further comprisingtransmitting 2 OFDM symbols during said signal field.
 6. The methodaccording to claim 1, further comprising, for a first spatial stream,transmitting a first long training sequence field, and a second longtraining field via said current spatial stream, in a system utilizing 2transmitted spatial streams, wherein said first long training sequencefield, and said second long training sequence field utilizes a longtraining sequence based on IEEE 802.11a.
 7. The method according toclaim 1, further comprising, for a second spatial stream, transmitting afirst long training sequence field, and a second long training field viasaid current spatial stream, in a system utilizing 2 transmitted spatialstreams, wherein said first long training sequence field, and saidsecond long training sequence field utilizes a phase shifted longtraining sequence based on IEEE 802.11a.
 8. The method according toclaim 1, further comprising, for a first spatial stream, transmitting afirst long training sequence field, a second long training sequencefield, and a third long training field via said current spatial stream,in a system utilizing 3 transmitted spatial streams, wherein said firstlong training sequence field, said second long training sequence field,and said third long training sequence field utilizes a multiplicativelyscaled long training sequence based on IEEE 802.11a.
 9. The methodaccording to claim 8, further comprising transmitting a first longtraining sequence field, a second long training sequence field, and athird long training field via a subsequent to said first spatial stream,in a system utilizing 3 transmitted spatial streams, wherein said firstlong training sequence field, said second long training sequence field,and said third long training sequence field utilizes a multiplicativelyscaled and phase shifted long training sequence based on IEEE 802.11a.10. The method according to claim 1, further comprising, for a firstspatial stream, transmitting a first long training sequence field viasaid current spatial stream, in a system utilizing 4 transmitted spatialstreams, wherein said first long training sequence field utilizes amultiplicatively scaled long training sequence based on IEEE 802.11a.11. The method according to claim 10, further comprising transmitting asubsequent to said first long training sequence field, via said firstspatial stream, wherein said subsequent long training sequence fieldutilizes a long training sequence based on IEEE 802.11a.
 12. The methodaccording to claim 11, wherein a subsequent to said first spatial streamcomprises a corresponding long training sequence to said first longtraining sequence field that utilizes said multiplicatively scaled longtraining sequence, and a plurality of long training sequence fields thatutilize said long training sequence based on IEEE 802.11a.
 13. Themethod according to claim 1, further comprising generating saidcorresponding one of said current plurality based on a discrete Fouriertransform matrix.
 14. A system for communicating information in amultiple input multiple output (MIMO) communications systems, the systemcomprising: a transmitter that generates a protocol data unit (PDU) fora current spatial stream comprising a current plurality of long trainingsequence fields; said transmitter appends a signal field subsequent to alast of said current plurality of long training sequence fields; saidtransmitter generates a PDU for a subsequent spatial stream comprising asubsequent plurality of long training sequence fields wherein one ofsaid subsequent plurality of long training sequence fields isorthonormal to a corresponding one of said current plurality of longtraining sequence fields; and said transmitter appends a subsequentsignal field subsequent to a last of said subsequent plurality of longtraining sequence fields.
 15. The system according to claim 14, furthercomprising antenna front end circuitry that transmits 2 orthogonalfrequency division multiplexing (OFDM) symbols during a first longtraining sequence field of said current plurality of long trainingsequence fields.
 16. The system according to claim 15, furthercomprising inverse fast Fourier transform circuitry that generates said2 OFDM symbols based on long training sequences that are approximatelyidentical.
 17. The system according to claim 15, wherein said antennafront end circuitry transmits 1 OFDM symbol during a long trainingsequence field that is subsequent to said first long training sequencefield.
 18. The system according to claim 14, further comprising antennafront end circuitry that transmits 2 OFDM symbols during said signalfield.
 19. The system according to claim 14, further comprising, for afirst spatial stream, antenna front end circuitry that transmits a firstlong training sequence field, and a second long training field via saidcurrent spatial stream, in a system utilizing 2 transmitted spatialstreams, wherein said first long training sequence field, and saidsecond long training sequence field utilizes a long training sequencebased on IEEE 802.11a.
 20. The system according to claim 14, furthercomprising, for a second spatial stream, antenna front end circuitrythat transmits a first long training sequence field, and a second longtraining field via said current spatial stream, in a system utilizing 2transmitted spatial streams, wherein said first long training sequencefield, and said second long training sequence field utilizes a phaseshifted long training sequence based on IEEE 802.11a.
 21. The systemaccording to claim 14, further comprising, for a first spatial stream,antenna front end circuitry that transmits a first long trainingsequence field, a second long training sequence field, and a third longtraining field via said current spatial stream, in a system utilizing 3transmitted spatial streams, wherein said first long training sequencefield, said second long training sequence field, and said third longtraining sequence field utilizes a multiplicatively scaled long trainingsequence based on IEEE 802.11a.
 22. The system according to claim 21,wherein said antenna front end circuitry transmits a first long trainingsequence field, a second long training sequence field, and a third longtraining field via a subsequent to said first spatial stream, in asystem utilizing 3 transmitted spatial streams, wherein said first longtraining sequence field, said second long training sequence field, andsaid third long training sequence field utilizes a multiplicativelyscaled and phase shifted long training sequence based on IEEE 802.11a.23. The system according to claim 14, further comprising, for a firstspatial stream, antenna front end circuitry that transmits a first longtraining sequence field via said current spatial stream, in a systemutilizing 4 transmitted spatial streams, wherein said first longtraining sequence field utilizes a multiplicatively scaled long trainingsequence based on IEEE 802.11a.
 24. The system according to claim 23,wherein said antenna front end circuitry transmits a subsequent to saidfirst long training sequence field, via said first spatial stream,wherein said subsequent long training sequence field utilizes a longtraining sequence based on IEEE 802.11a.
 25. The system according toclaim 24, wherein a subsequent to said first spatial stream comprises acorresponding long training sequence to said first long trainingsequence field that utilizes said multiplicatively scaled long trainingsequence, and a plurality of long training sequence fields that utilizesaid long training sequence based on IEEE 802.11a.
 26. The systemaccording to claim 14, wherein said transmitter generates saidcorresponding one of said current plurality based on a discrete Fouriertransform matrix.
 27. A method for communicating information in amultiple input multiple output (MIMO) communications system, the methodcomprising: constructing a mixed mode protocol data unit (PDU)comprising, a legacy short training sequence field comprising a timeduration of approximately 8 microseconds; appending a legacy longtraining sequence field comprising a time duration of approximately 8microseconds, subsequent to said legacy short training sequence field;appending a legacy signal field comprising a time duration ofapproximately 4 microseconds, subsequent to said legacy long trainingfield; appending a high throughput signal field comprising a timeduration of approximately 8 microseconds, subsequent to said legacysignal field; appending a high throughput short training sequence fieldcomprising a time duration of approximately 3.2 microseconds, subsequentto said high throughput signal field; and appending a plurality of longtraining sequence fields, comprising a time duration of approximately 4microseconds, subsequent to said high throughput short training sequencefield.